MAC: syscalls: Split mac_set_proc() into reusable pieces

This is in preparation for enabling the new setcred() system call to set
a process' MAC label.

No functional change (intended).

MFC after: 2 weeks
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46905

(cherry picked from commit 8a4d24a39098ed8170a37ca2aa83bf1da1976de1)